Our grandchildren will be reading about the year 2010 in their history textbooks. If the polls are to be believed, the nation is heading into a constitutional crisis, out of which a better politics could be born. The Liberal Democrats are on the verge of holding real power for the first time in 80 years. Nick Clegg is determined to use it to reform the political system. Finally, proportional representation – seen by John Stuart Mill as the "sheet anchor of democracy" – is on the cards.

The liberal surge has shaken the establishment to its foundations. The Liberal Democrats, a party that owes nothing to big business, the Murdoch press or the trade unions, could form the vanguard of a great, reforming parliament. You only need to look at the desperate attempts of the rightwing press to uncover some past indiscretion on Clegg's part to see what a revolutionary figure he could be. His only debt will be to the electorate. Clegg has argued that this is a "liberal moment". Sadly, that's probably not true. But it is certainly a democratic moment.

This is why so many people now see the Liberal Democrats as the torch bearers for the progressive cause. The writers, thinkers, artists and academics who signed today's letter in the Guardian are by no means all card-carrying Liberal Democrats. There are some, like John Kampfner, the former editor of the New Statesman, who made a public leap to the Liberal Democrats – in a pamphlet for CentreForum titled Lost Labours – long before Clegg turned the election campaign inside out. But what excites all of us, as individuals, is not the victory or defeat of any particular party, but the possibility of genuine reform. This election offers a rare, precious opportunity to end the two-tribes-go-to-war model of politics for good.

There are plenty more people out there, too. For each person who signed up, there were at least two more who were supportive but who decided that for now, discretion was the better part of valour. They remain in the closet, but they are for Clegg.

For those who have "come out" for Clegg, the moment of conversion differs. For some, it was his party's principled stance against the Iraq war; for others, Clegg and Vince Cable's refusal to enter the bidding war between the other two parties on cuts to their inheritance tax. Clegg's authentic outburst against Cameron's allies on the continent as a "bunch of nutters" pleased quite a few signatories, too. The party's green agenda appeals to many.

For the last 50 years, it has been a staple assumption of British political life that a vote for the Liberal Democrats – whatever their many attractions – was simply a "waste", or a gift to the Conservatives under our antiquated first-past-the-post system. Clegg has shattered this assumption in a matter of days. Although all the psephological calculations have gone haywire, it seems clear that the Tories are being hit hardest by the rise in Liberal Democrat support. Cameron's hopes of a working majority are shrinking, hence his frantic backpedalling on electoral reform and urgent discussions with the nationalists in Scotland and Wales.

Of course, people in marginal seats will be making their own decisions about the best way to ensure a reforming parliament. The parliamentary majority required in the Commons after 6 May is a majority of pluralists – of MPs committed to deep political reform, willing to work with pluralists in other parties, and ready to face down the tribalists and conservatives on all sides.

If – and only if – the Liberal Democrats put in a strong performance, politics as usual will end. Clegg will be the progressive conscience of the next parliament. Some people, good liberal progressives, are waiting to see how the votes are counted on 6 May. But for some of us, sitting this one out is not an option. When our grandsons and granddaughters ask – "so what did you do in May 2010?" – we want to be able to say that at this moment, at the potential birth of a new democracy, we tried to play our part.

Richard Reeves is the author of John Stuart Mill – Victorian Firebrand